1. Field of the Invention
The present invention relates to an ultra narrow band laser apparatus for producing ultra narrow band laser light from a fluorine laser for use as a light source for a stepper or other fluorine exposure apparatus, and to a fluorine exposure apparatus.
2. Description of the Related Art
Qualities required of exposers for use in lithography include resolution, precision in alignment, processing capability, and apparatus reliability. Of these, resolution R, which is most intimately related to fine patterning ability, is expressed by R=kxc2x7xcex/NA (where k is a constant, xcex is the exposure wavelength, and NA is the numerical aperture of the projecting lens). Accordingly, lower exposure wavelengths xcex are more useful in terms achieving better resolution.
Conventional exposers utilize exposer light sources such as the i line (wavelength 365 nm) of a silver mercury lamp or a 248 nm-wavelength krypton-fluoride (KrF) excimer laser. These are respectively called i line exposers and KrF exposers. Reducing projection lens assemblies composed of a multitude of lenses comprising quartz glass are widely used as projection optical systems in these i line exposers and KrF exposers.
To enable processing of smaller features, exposers employing 193 nm-wavelength argon-fluoride (ArF) excimer lasers as light sources are coming into use as the next generation of lithography exposers. These are termed ArF exposers; these ArF exposers employ narrow band ArF excimer lasers having spectral width (bandwidth) of about 0.6 pm. For the reducing chromatic abberation, achromatic lenses comprising dual materials are used.
Narrow band elements known to provide ArF excimer laser bandwidths as small as about 0.6 pm include etalons, gratings, mode selectors, and other such elements. Of these elements, mode selectors are discussed in some detail in PROCEEDING OF THE EEE, VOL. 60, NO. 4, APRIL 1972, pp. 422-441.
ArF excimer laser apparatus include those employing two laser units. Specifically, a process termed injection seeding, wherein one of the laser units generates a seed light and this seed light is injected into an oscillator (the second laser unit), is implemented in an ArF excimer laser apparatus.
An injection seeding type ArF excimer laser apparatus is discussed, for example, in xe2x80x9cDai 59-kai Oyo Butsurigaku Kanko Rengo Koenkai Preprintsxe2x80x9d, p. 950, 17-a-P2-1, 1998.
A fluorine exposer employing as the light source an approximately 157 nm-wavelength fluorine laser is under study as a next-generation ArF exposer for lithography.
This fluorine laser produces two intense oscillation lines of different wavelengths and light intensities (also called output lines), the wavelengths of which are 157.6299 nm (wavelength xcex1) and 157.5233 nm (wavelength xcex2) respectively. Bandwidth of each of the two oscillation lines is from 1 to 2 pm.
When using this fluorine laser for exposure, it is typically advantageous to select the stronger wavelength line (xcex1=157.6299 nm) (hereinbelow termed the xe2x80x9cstrong line,xe2x80x9d the other being termed the xe2x80x9cweak linexe2x80x9d) (this process is hereinbelow termed xe2x80x9csingle line modexe2x80x9d). Conventionally, one or two prisms were used for single line mode.
Experimental findings pertaining to single line mode in fluorine lasers for use in fluorine exposers are reported, for example, in xe2x80x9cSPIE 24th International Symposium on Microlithography, February, 1999.
Double line mode for a fluorine laser is described, for example, in CAN. J. PHYS. VOL. 63, 1983, pp. 217-218.
In the fluorine exposers described above, it is a difficult matter to implement a refracting type reducing projection optical system using simply the lenses typically employed in exposers to date (i.e., exposers up through ArF exposers).
The reason is that with a 157 nm wavelength fluorine laser, transmittance through quartz glass is extremely low, imposing severe limitations on the materials that can be used to, for example, calcium fluoride. When a reducing projection optical system is constructed of monochromatic lenses of calcium fluoride only, when the fluorine laser is tuned to single line mode, the oscillation laser light from the fluorine laser will not have a sufficiently narrow band. The bandwidth resulting from band narrowing is about 1 pm, but in actual practice band narrowing to a bandwidth about ⅕ of that, namely, a bandwidth of about 0.2 pm, is thought necessary for single line mode.
Conventionally, since it has proven difficult to achieve band narrowing to bandwidths of 0.2 pm or smaller for single line mode for a fluorine laser, it has been thought necessary to implement the reducing projection optical system with a reflecting/refracting type reducing projection optical system (hereinbelow referred to as a catadioptric type) capable of being used over a bandwidth 10 times wider than a total reflection type optical system composed of lenses only.
The reason why it has been difficult in conventional practice to achieve band narrowing of fluorine lasers to bandwidths of 0.2 pm or smaller is that when one or two prisms are situated in the laser resonator for single line tuning, laser output drops down to about 40%. Installing an etalon or the like enabling greater band narrowing (i.e., one with reflectivity on the order of 80%) in order to achieve band narrowing of the bandwidth to 0.2 pm increases the insertion loss further by about 50%. This makes laser operation difficult or appreciably reduces laser output.
The reasons for the significant drop in laser output occurring with installation of an etalon in a fluorine are now discussed.
It is known that for an etalon having a high reflectivity reflective film, a low degree of planarity typically results in lower maximum transmittance. Where the etalon is fabricated with a calcium fluoride substrate, a typical optical system capable of being used at 157 nm wavelength, the etalon has a lower degree of hardness than quartz, and is moreover crystalline, which makes polishing difficult; for these and other reasons, planarity of no less than about {fraction (1/20)} the wavelength can be achieved. On the other hand, it is known that, with the use of quartz, etalons affording planarity on the order of {fraction (1/100)} the wavelength can be utilized.
Thus, where, for example, an etalon with finesse of 10 is used for band narrowing of 2 pm bandwidth laser light to a bandwidth of 0.2 pm, it is necessary for the etalon to have a coating with reflectivity of 80% or above. If the degree of planarity of the etalon is {fraction (1/20)} the wavelength, maximum transmittance on the order of only about 50% can be achieved in the etalon.
Accordingly, it is a first object of the present invention to provide an ultra narrow band fluorine laser apparatus capable of operation in single line mode, with the bandwidth of the line narrowed to about 0.2 pm, and additionally affording a reduction in the drop in laser output.
It is a second object of the invention to provide an ultra narrow band fluorine laser apparatus whereby oscillation laser light from a fluorine laser may be provided as an exposure light source to a fluorine exposer utilizing a lens-only total refraction type reducing projection optical system.
In systems where a single line of a fluorine laser (i.e., the line of wavelength xcex1=157.6299 nm) is used as-is, the line spectrum is determined absolutely spectrally, so wavelength stabilization is not needed. Where bandwidth is narrowed to about 0.2 pm, despite the need for the band narrowed wavelength to be stable within a 1 to 2 pm bandwidth single line spectrum, it is difficult to ascertain whether wavelength is in fact stable. This makes it difficult to correctly calibrate wavelength.
The reason is that in the vacuum ultraviolet region in proximity to the 157.6299 nm xcex1 wavelength, it was difficult to use an absolute wavelength where the wavelength had been narrowed to about 0.1 pm (another stable light source (lamp) or absorption line).
It was also difficult to develop a fluorine exposer comprising a fluorine laser apparatus capable of output of single line-tuned, band narrowed laser light.
It is accordingly a third object of the invention to provide a fluorine exposure apparatus and ultra narrow band fluorine laser apparatus capable of correct calibration of the wavelength of laser light from a fluorine laser wherein, for example, the 157.6299 nm xcex1 wavelength line of bandwidth of 1 to 2 pm has been narrowed to about 0.2 pm.
In order to achieve the first object, a first invention provides an ultra narrow band fluorine laser apparatus which provides oscillated laser light of a fluorine laser as a light source for an exposure apparatus, comprising:
a wavelength selection element whose transmittance or reflectivity varies cyclically in accordance with a wavelength of incident light, for narrowing a band of the oscillated light of the fluorine laser, wherein
the wavelength selection element is composed of:
an optical element whose transmittance or reflectivity varies cyclically, so that, of two oscillation lines of different wavelengths and light intensities in the fluorine laser, when a center wavelength of a first oscillation line thereof having a stronger light intensity is situated at one selected wavelength in the element, a center wavelength of a second oscillation line having a weaker light intensity than the first oscillation line is situated between two adjacent selected wavelengths in the element.
In a second invention according to the first invention, wherein the wavelength selection element is constituted such that transmittance at the center wavelength of the second oscillation line becomes 0.64 times or less the transmittance at the center wavelength of the first oscillation line.
In a third invention according to the first invention, the wavelength selection element is a mode selector composed of splitting means having a beam splitting face and two reflection means having reflecting faces.
In a fourth invention, the ultra narrow band fluorine laser apparatus according to the first or second invention further comprises an oscillating stage for oscillating the laser light of the fluorine laser; and an amplifying stage, and wherein
the wavelength selection element is situated on an optical path between the oscillating stage and the amplifying stage.
In order to achieve the second object, according to a fifth invention, in any of the first to fourth inventions, wherein the laser light band-narrowed by the wavelength selection element is provided to a fluorine exposure apparatus having a lens-only total refraction type reducing projection optical system.
The first and second inventions shall now be described making reference to FIGS. 1 and 2.
Referring to FIG. 1, an etalon 16, a wavelength selection element serving as a band narrowing element, is situated on the optical path between a beam splitter 14 and a mirror 15. The cycle (FSR) of etalon 16 is 3.0 pm; finesse is 15.
Referring to FIG. 2, maximum transmission wavelength xcexa is matched to the center of the 157.6299 nm xcex1 wavelength strong line (oscillation line) L1 by etalon 16. As a result, laser beam L12 incident on etalon 16 from beam splitter 14, upon passing through etalon 16 (i.e., laser beam L13), assumes peak power of about 50% at the center wavelength of strong line L1, with bandwidth being about 0.2 pm, {fraction (1/15)} the original line width.
Dividing the wavelength differential of the two lines of the fluorine laser of 106.6 pm (=157,629.9 pmxe2x88x92157,523.3 pm) by the FSR of 3.0 pm gives a value of 35.53, so transmittance at the 157.5233 nm xcex2 wavelength is about several %.
That is, since the decimal part of the value 106.6/FSR (35xc2x753) is 0.53, in etalon 16 affording maximum transmittance at the 157,629.9 nm xcex1 wavelength, the 157,523.3 nm xcex2 wavelength is substantially medial with respect to two adjacent maximum transmission wavelengths xcexb, xcexc, as shown in FIG. 2. Thus, since etalon 16 produces large loss for the 157.5233 nm xcex2 wavelength weak line (oscillation line) L2, laser beam L13 has a strong line L1 component of 90% or greater with bandwidth of about 0.2 pm.
Returning now to FIG. 1, as laser beam L13 is reflected by mirror 15, again passing through etalon 16 to be reflected by beam splitter 14 and directed into a laser chamber 13, whereby the proportion of the band narrowed strong line L1 increases further. Accordingly, the laser is oscillated with the strong line L1 exclusively, whereby a laser beam L14 of 0.2 pm bandwidth strong line L1 exclusively is obtained at output mirror 11.
The intensity ratio of the two lines L1, L2 of the fluorine laser is the to be such that the weak line L2 is about ⅙ to {fraction (1/7)} the strong line L1. In consideration of this intensity ratio, in order that extremely weak light traverse at least twice the etalon or other wavelength selection element so that intensity at the center wavelength of weak line L2 drops to 1% or less the intensity at the center wavelength of strong line L1, the transmittance ratio at the wavelength selection element is appropriately one such that transmittance at the center wavelength of weak line L2 is (⅙){circumflex over ( )}(xc2xc)=0.64 times or less the transmittance at the center wavelength of strong line L1.
That is, characteristics (specifications) for the wavelength selection element are established such that transmittance at the center wavelength of weak line L2 is about 0.64 times or less the transmittance at the center wavelength of strong line L1.
According to the first invention described hereinabove, the wavelength selection element is composed of an optical element with cyclically varying transmittance or reflectivity, such that, of the two oscillation lines of different wavelengths and light intensities in the fluorine laser, when the center wavelength of the oscillation line having stronger light intensity is situated at one selected wavelength in the element, the center wavelength of the oscillation line having weaker light intensity is situated between two adjacent selected wavelengths in the element, whereby it becomes possible to generate efficiently laser light of bandwidth narrowed to about 0.2 pm with a single strong line of 157.6299 nm wavelength exclusively, without the use of prisms to give the single line.
According to the second invention, transmittance at the center wavelength of the second oscillation line (weak line L2) is set to about 0.64 times or less the transmittance at the center wavelength of the first oscillation line (strong line L1), whereby oscillation of laser light by the second oscillation line may be inhibited.
The third invention is now described making reference to FIG. 5.
Referring to FIG. 5, in ultra narrow band fluorine laser apparatus 500, the resonator that surrounds laser chamber 52 is composed of a total reflection mirror 51 and a mode selector 501. Mode selector 501 is composed of a beam splitter 53 having a beam splitting action and two reflecting mirrors 54a, 54b. This mode selector 501 is the wavelength selection element having a function like that of an output mirror for outputting laser light of the selected wavelength.
In mode selector 501, where d is the gap (more accurately, the optical path length) between reflecting mirror 54a and reflecting mirror 54b, the mode selector 501 cycle (FSR) is expressed as xcex{circumflex over ( )}2/(2nd), where n=1.
Here, since d=12.3 mm, FSR is 1.01 pm; as a result, 106.6/1.01=105.5.
In mode selector 501 upon which a laser light impinges from laser chamber 52, a portion of this laser light is passed through beam splitter 53, while another portion of the laser light is reflected by beam splitter 53 and then reflected by reflecting mirror 54b. 
A portion of the laser light reflected by reflecting mirror 54b is reflected by beam splitter 53 or passes through beam splitter 53. The laser light reflected by beam splitter 53 is again directed into laser chamber 52, while laser light passing through beam splitter 53 is reflected by reflecting mirror 54a. 
Laser light reflected by reflecting mirror 54a is then reflected by beam splitter 53 and output as a laser beam L50; a portion of this laser light passes through beam splitter 53 and is reflected by reflecting mirror 54b. 
By repeating this operation, as laser light is directed into laser chamber 52 the proportion of narrow band strong line L1 in the laser light increases, which is then output from beam splitter 52.
That is, since 106.6/1.01=105.5, tuning to the 157.6299 nm xcex1 wavelength strong line L1 by mode selector 501 suppresses the 157.5233 nm xcex2 wavelength weak line L2, whereby a 157.6299 nm xcex1 wavelength laser beam L50 is obtained from beam splitter 53.
According to the third-invention described hereinabove, the mode selector serving as the wavelength selection element is composed of splitting means having a beam splitting face (a beam splitter, for example) and two reflecting means having reflecting faces (reflecting mirrors, for example), whereby the means having a beam splitting face may be constituted of a no-coat (no coating) substrate, and mirrors with total reflection films may be employed as the means having reflecting faces.
Accordingly, the wavelength selection element may be constituted without the use of a half-mirror employing a partial reflection layer for an etalon (one wavelength selection element).
That is, since the need to use a wavelength selection element, such as an etalon, requiring a partial reflection layer (which is susceptible to damage) is obviated, in an ultra narrow band fluorine laser apparatus employing a mode selector, the mode selector remains undamaged and moreover is stable for an extended period, affording band narrowing of oscillation laser light.
The fourth invention is now discussed making reference to FIG. 7.
Referring to FIG. 7, ultra narrow band fluorine laser apparatus 700 is of seeded injection type, composed of a seed laser 71 as the oscillating stage and an oscillator 72 as the amplifying stage.
As regards seed laser 71, an output mirror 73 and a total reflection mirror 74 are situated to either side of a laser chamber 75 so as to constitute a stabilizing resonator. No band narrowing element is present in the resonator. Accordingly, the laser beam L71 from seed laser 71 contains both strong and weak lines L1, L2, with both lines, being un-narrowed, having bandwidth of about 1 pm.
Laser beam 71 passes through an etalon 76, a wavelength selection element situated to the outside of seed laser 71. The characteristics of etalon 76 are an FSR of 3.0 pm and finesse of 15. The laser beam L72 passing through etalon 76 is narrowed to bandwidth of 0.2 pm and consists of a single line only.
The energy of laser beam L72 is about {fraction (1/10)} lower than that of laser beam L71. The narrowed laser beam L72 proceeds to an oscillator 72, the second fluorine laser apparatus. This laser beam L72 is injected as seed light into the resonator via the aperture in an apertured concave mirror 78. As laser beam L72 discharges during passage through laser chamber 79, there is obtained a laser beam L73 having the same bandwidth but increased power.
According to the fourth invention described hereinabove, a wavelength selection element is situated between the oscillating stage and the amplifying stage, whereby the laser light need not be subjected to band narrowing in the oscillating stage, thus facilitating laser oscillation in the oscillating stage to give laser light of sufficiently long pulse width.
Accordingly, with the ultra narrow band fluorine laser of the fourth invention, laser light may be amplified highly efficiently even with a modicum of synchronization error between the oscillating stage and the amplifying stage.
The fifth invention is now discussed making reference to FIG. 3.
Referring to FIG. 3, fluorine exposer 300 is broadly composed of an exposer main body 200 and an ultra narrow band fluorine laser apparatus 100 (see FIG. 1).
Exposer main body 200 is arranged on a grating 21 in a cleanroom, while ultra narrow band fluorine laser apparatus 100 is arranged on a floor bed 22 (typically termed a xe2x80x9csubfloorxe2x80x9d) situated below grating 21.
The laser beam L20 obtained from ultra narrow band fluorine laser apparatus 100, which consists exclusively of the strong line L1 with bandwidth of approximately 0.2 pm, is reflected upward by a mirror 23a so as to pass through an aperture 24 in grating 21 and into exposer main body 200.
In exposer main body 200, the laser beam L22 from a reticle 29 passes through a reducing projection lens 30 and impinges on a wafer 31. The reducing projection lens 30 used as the reducing projection optical system is composed of a monochromatic lens comprising calcium fluoride.
According to the fifth invention described hereinabove, the laser beam from an ultra narrow band fluorine laser apparatus is provided to a fluorine exposure apparatus having a total refraction type reducing projection optical system, whereby a reducing projection optical system design analogous to that in a conventional krypton-fluoride (KrF) exposer may be adopted in a fluorine exposure apparatus, thus quickly and inexpensively providing a commercial fluorine exposure apparatus.
In order to achieve the third object, a sixth invention provides an ultra narrow band fluorine laser apparatus which narrows a band of laser light of a fluorine laser and provides the band-narrowed laser light as a light source for an exposure apparatus, comprising:
a wavelength selection element being arranged so that a selected wavelength can be adjusted, for narrowing the band of incident laser light of the fluorine laser for output;
monitoring means for monitoring the output of laser light output from the wavelength selection element; and
adjusting means for adjusting, on the basis of a monitoring outcome from the monitoring means, the wavelength selected by the wavelength selection element so as to maximize the output of laser light output from the wavelength selection element.
In a seventh invention according to the sixth invention, the wavelength selection element comprises:
an optical element for varying a selected wavelength in accordance with an angle of incidence of the laser light thereon;
the monitoring means comprises:
means for monitoring output of laser light of each selected wavelength corresponding to each of a plurality of different angles of incidence for the laser light incident on the optical element; and
the adjusting means comprises:
control means for computing, on the basis of a monitoring outcome from the means for monitoring, the angle of incidence serving to maximize the output of the laser light output from the wavelength selection element, and causing laser light to be incident on the wavelength selection element on the basis of the angle of incidence so calculated.
In an eighth invention, the ultra narrow band fluorine laser apparatus according to the seventh invention further comprises:
varying means for varying a placement position of the wavelength selection element within a range in which the laser light is incident on the wavelength selection element; and
the control means comprises:
means for controlling the varying means in such a way that when the output of laser light output from the wavelength selection element whose placement position is varied by the varying means reaches maximum, laser light is caused to be incident on the wavelength selection element on the basis of the angle of incidence calculated from the monitoring outcome.
In a ninth invention, the laser apparatus according to the seventh invention further comprises:
reflecting means for reflecting laser light so as to guide the laser light into the wavelength selection element; and
varying means for varying a placement position of the reflecting means within a range such that laser light reflected by the reflecting means is incident on the wavelength selection element; and
the control means comprises:
means for controlling the varying means in such a way that when the output of the wavelength selection element upon which is incident laser light from the reflecting means whose placement position is varied by the varying means reaches maximum, laser light is caused to be incident on the wavelength selection element on the basis of the angle of incidence calculated from the monitoring outcome.
In a tenth invention according to the sixth invention, the wavelength selection element is a mode selector composed at least of splitting means having a beam splitting face; and two reflecting means having reflecting faces, an optical path length being determined by relative positions of these plurality of composing elements;
the monitoring means comprises:
means for monitoring output of laser light for each of selected wavelengths corresponding to each of a plurality of different optical path lengths in the mode selector; and
the adjusting means comprises:
optical path length adjusting means for adjusting optical path length in the mode selector by varying the placement of at least one composing element selected from the plurality of composing elements in the mode selector; and
control means which, for each of the plurality of different optical path lengths resulting from adjustment by the optical path length adjusting means, calculates, on the basis of a monitoring outcome monitored by the means for monitoring, the optical path length so as to maximize the output of the laser light output from the wavelength selection element.
The fluorine exposure apparatus of an eleventh invention comprises the ultra narrow band fluorine laser apparatus according to any of the sixth to tenth inventions; and
an exposure apparatus main body employing narrow band laser light oscillated from the ultra narrow band fluorine laser apparatus as a light source for exposure to subject a wafer to an exposure process, and notifying the ultra narrow band fluorine laser apparatus when a wafer to be subjected to the exposure process is exchanged, wherein
the ultra narrow band fluorine laser apparatus is designed to adjust a wavelength selected by the wavelength selection element when notified by the exposure apparatus of exchange of the wafer to be subjected to the exposure process.
The fluorine exposure apparatus of a twelfth invention comprises the ultra narrow band fluorine laser apparatus according to any of the sixth to tenth inventions; and
an exposure apparatus main body employing narrow band laser light oscillated from the ultra narrow band fluorine laser apparatus as a light source for exposure to subject a wafer to an exposure process; wherein
the ultra narrow band fluorine laser apparatus is designed to adjust a wavelength selected by the wavelength selection element immediately after start of laser oscillation.
The sixth to eighth inventions shall be described making reference to FIGS. 9 and 10.
A mechanism operates at predetermined time intervals to calibrate laser beam L91 wavelength to the center of strong line L1.
When a wafer is not being subjected to an exposure process (when a wafer is not being irradiated with laser light), a power monitor 99 detects the output of laser beam L92 from a mirror 98 that has come to a halt at a location indicated by symbol 98b, and sends a signal reflecting this finding (monitoring outcome) to a control unit 102 via a signal line 101a. 
On the basis of this signal, control unit 102 performs rotation control of a rotating stage 103 via a signal line 101b. That is, while rotating an etalon 97 in small increments by performing rotation control of a rotating stage 103, control unit 102 measures the output of laser beam L92.
This yields a characteristics graph (profile curve) of measured output values (relative values) versus etalon 97 setting angle like that shown in FIG. 10. Setting angle corresponds to the angle of rotation when rotating stage 103 is rotated from a pre-established reference position for etalon 97. Setting angle also corresponds to the angle of incidence of laser light from beam splitter 95 onto etalon 97.
Control unit 102 computes a setting angle xcex8 corresponding to a medial location between the sloping lines on either side of the profile curve (peak waveform), for example, to a medial point P lying midway between point P1 and point P2. By rotating the rotating stage 103, i.e., etalon 97, to give this setting angle xcex8, the wavelength of laser beam L91 is matched with the center wavelength of strong line L1.
According to the sixth to eighth inventions described hereinabove, by monitoring laser light output (output characteristics) by the wavelength selection element, it is possible to adjust the wavelength selected by the wavelength selection element (for example, adjusting the position of the wavelength selection element) in order to maximize the output of laser light from the wavelength selection element.
Accordingly, it is possible to reset the wavelength selection element to a state such that laser light output is maximized prior to actual use of the laser light as exposure light.
By periodically re-adjusting the position of the wavelength selection element through monitoring of wavelength selection element laser light output so as to constantly maintain that laser light output at maximum, it becomes possible to calibrate the wavelength of narrow band laser light.
The ninth invention shall be described making reference to FIGS. 10 and 11.
When a wafer is not being subjected to an exposure process, a power monitor 110 detects the output of laser beam L102 from a mirror 109 that has come to a halt at a location indicated by symbol 109b, and sends a signal reflecting this finding (monitoring outcome) to a control unit 112 via a signal line 111a. On the basis of this signal, control unit 112 controls a piezo element 113 via a signal line 111b to slightly tilt a total reflection mirror 106.
Thus, when total reflection mirror 106 is tilted to a tilt angle based on the signal from control unit 112, the optical path of the laser light passing through etalon 108 changes slightly with this tilting (for example, the angle of incidence of laser light from total reflection mirror 106 onto etalon 108 changes) so that maximum transmission wavelength in etalon 108 changes slightly.
In control unit 112, the output value of laser light L102 versus the angle of incidence of laser light from total reflection mirror 106 onto etalon 108 is measured.
This yields a profile curve (see FIG. 10) of measured output values (relative values) versus the angle of incidence of laser light onto etalon 108, i.e., the angle of tilt of total reflection mirror 106. On this curve, the vertical axis gives measured output values (relative values) and the horizontal axis gives the tilt angle of total reflection mirror 106.
Once such a profile curve has been generated, control unit 112 computes a total reflection mirror 106 tilt angle corresponding, for example, to a medial point P lying midway between point P1 and point P2 on the profile curve (peak waveform). By controlling piezo element 113 in order to change the tilt of total reflection mirror 106 so as to give this tilt angle, the wavelength of laser beam L101 is matched with the center wavelength of strong line L1.
According to the fourth invention described hereinabove, by monitoring laser light output by the wavelength selection element (output characteristics) it is possible to adjust the wavelength selected by the wavelength selection element (for example, adjusting the position of the wavelength selection element) in order to maximize the output of laser light from the wavelength selection element, thus enabling calibration of the wavelength of narrow band laser light.
The tenth invention shall be described making reference to FIGS. 10 and 12.
When a wafer is not being subjected to an exposure process, a power monitor 110 detects the output of laser beam L102 from a mirror 109 that has come to a halt at a location indicated by symbol 109b, and sends a signal reflecting this finding to a control unit 112 via a signal line 111a. 
On the basis of the received signal, control unit 112 controls movement of a piezo element 113 via a signal line 111b to bring about slight forward or backward movement of a reflecting mirror 122b. This movement changes the gap between a reflecting mirror 122a and reflecting mirror 122b (i.e., optical path length) and thus the selected wavelength in mode selector 120 changes.
In control unit 112, the output value of laser light L102 versus the distance of movement of reflecting mirror 122b from a predetermined location is measured.
This yields a profile curve (see FIG. 10) of measured output values (relative values) versus the distance of movement of reflecting mirror 122b (optical path length of reflecting mirror 122a and reflecting mirror 122b) in mode selector 120. On this curve, the vertical axis gives measured output values (relative values) and the horizontal axis gives values showing the distance of movement of reflecting mirror 122b. 
Once such a profile curve has been generated, control unit 112 computes a value showing the distance of movement of reflecting mirror 122b corresponding, for example, to a medial point P lying midway between point P1 and point P2 on the profile curve (peak waveform). By controlling piezo element 113 in order to move reflecting mirror 122b so as to achieve this distance of movement value, the wavelength of laser beam L101 is matched with the center wavelength of strong line L1.
According to the tenth invention described hereinabove, even where a mode selector is used as a wavelength selection element, since the wavelength selected by the mode selector is adjusted on the basis of monitoring of laser light output by the mode selector (output characteristics), it is possible to reset the wavelength selection element to a state such that laser light output is maximized prior to actual use of the laser light as exposure light.
The eleventh invention shall be described making reference to FIGS. 10 and, 13.
In ultra narrow band fluorine laser apparatus 900 of fluorine exposer 1300, during the interval of about 20 seconds from completion of exposure treatment of a wafer 141 to completion of placement and alignment of a next wafer on stage 142, wavelength is calibrated on the basis of a profile curve (see FIG. 10).
As regards the timing for the wavelength calibration, a signal indicating completion of exposure treatment of the wafer is transmitted to ultra narrow band fluorine laser apparatus 900 via a signal line 143. Ultra narrow band fluorine laser apparatus 900, having received the exposure completion signal, acquires the profile curve and performs wavelength calibration on the basis of this curve.
The twelfth invention shall be described making reference to FIG. 14.
In ultra narrow band fluorine laser apparatus 900 of fluorine exposer 1300, several tens of pulses generated at a time T1 at which initial effect is produced (hereinbelow referred to as xe2x80x9cinitial effect timexe2x80x9d), shown in FIG. 14, are utilized to calculate a curve like that shown in FIG. 15, and wavelength calibration is performed on the basis of this curve.
According to the eleventh and twelfth inventions described hereinabove, laser light wavelength is calibrated each time that a wafer for exposure treatment is exchanged, thus avoiding a state in which exposure conditions differ for individual wafers. That is, exposed wafers of uniform quality are obtained.
Further, as wavelength calibration of laser light is performed utilizing pulses generated at the outset of laser operation, wavelength-calibration narrow band laser light is used for wafer exposure.